Class D amplifiers, comprising switching power stages are used in many applications because of their efficiency. The output signal of such an amplifier is a pulse-width modulated (PWM) signal, which is low-pass filtered for obtaining the amplified analog signal. To reduce the high-frequency content, and simplify filtering, multi-level PWM signals, which more closely represent the input signal may be used. To drive the load with a multi-level PWM signal an output stage that can reproduce multiple output levels is required. The amount of power that can be delivered to the loudspeaker load depends on the available supply voltage and the loudspeaker load impedance. The maximum sinewave power that can be delivered to the load is calculated in equation 1 below, where Vs represents the supply voltage, which sets the maximum amplitude of the sinewave and Rload the load impedance.
                    Pload        =                              V            s            2                                2            ⁢            Rload                                              (        1        )            
To increase the amount of power that can be delivered to the load the supply voltage can be increased or the load impedance lowered or a combination of both options can be used. The loudspeaker load impedance cannot be made infinitely small and the available supply voltage is often fixed. This especially holds for portable devices where the supply is a battery with a given cell voltage. To increase the amount of power that can be delivered to the load the supply voltage must be boosted. The switching power-stage of a class-D amplifier is then operated from the boosted power supply with a voltage value of NVs as shown in FIG. 1. When the supply voltage is e.g. doubled i.e. N=2, then the maximum power that can be delivered to the loudspeaker load is quadrupled. A common way to boost the supply voltage is to use an inductive DC/DC converter. But a coil is a highly undesirable component. It is expensive and has a large footprint. A capacitive DC/DC converter to boost the supply voltage is therefore preferable.
A known circuit using capacitors for boosting the supply voltage is a charge-pump. This capacitive voltage-doubler circuit has two states and is continuously switched between both states by operating the switches. In a first state, a capacitor C0 is charged to the supply voltage Vs. While capacitor C0 is charged, a buffer capacitor Cb delivers current to a load and it is discharged. Because a capacitor in parallel with the load Cb is discharged the output voltage drops. After the output voltage has dropped to a certain value, the circuit is switched to a second state. Capacitor C0 is connected in series with the supply voltage. Because of some charge redistribution between C0 and Cb the output voltage will be lower than 2Vs. After switching both capacitors C0 and Cb are discharged and the output voltage drops. After the output voltage has dropped below a certain value, the circuit is switched again to the previous state. Two different slopes in the output voltage can be distinguished. In the first state only buffer capacitor Cb supplies all the load current resulting in a faster drop in the output voltage compared to the second state where the load current is supplied by capacitor Cb and C0 in parallel. A resistance in series with capacitor Cb would cause an additional voltage step in the output voltage when the capacitive voltage-doubler circuit is switched from the second state to the first state caused by the additional load current which capacitor Cb has to deliver in the first state.
To avoid this problem a double-phase capacitive circuit as is shown in FIG. 2 may be used. In FIG. 2, the above-mentioned notations can be easily observed. The double-phase capacitive circuit has also two states and it is continuously switched between both states by operating the switches. In the double-phase capacitive voltage-doubler circuit two capacitors, C0 and C1 are switched. In the first state the capacitor C0 is charged to the supply voltage and capacitor C1 is connected in series with the supply and in parallel with capacitor Cb. In the second state the function of capacitor C0 and C1 are interchanged. Now capacitor C1 is charged to the supply and capacitor C0 is connected in parallel with capacitor Cb. In both states there is a capacitor in parallel with Cb and the slope of the output voltage drop is equal in both states.